1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube having an improved structure of a frame and an inner shield for supporting a shadow mask.
2. Description of the Related Art
A cathode ray tube (CRT) is an imaging apparatus which displays a predetermined image by scanning an electron beam onto fluorescent material coated on the surface of a panel. CRT is widely used for a television set, a computer monitor, an oscilloscope, etc.
FIG. 1 is a sectional view of a typical CRT. Referring to the drawing, a CRT 1 includes a panel 10 having a fluorescent film 11 formed on the inner surface thereof, a funnel 30 coupled to the panel 10 forming a seal, a shadow mask 12 installed at a predetermined interval from the fluorescent film 11, a frame 20 for supporting the shadow mask 12, a hook spring 23 for supporting the frame 20 with respect to the panel 10, and a stud pin 24. Also, the CRT 1 has an electron gun 33 installed at a neck portion 31 of the funnel 30 and a deflection yoke 34 installed at a cone portion 32 of the funnel 30. Reference numeral 22 represents an inner shield.
FIG. 2 is a cross-sectional view showing a portion of the CRT of FIG. 1 in detail. In the drawing, the frame 20 having an "L" shaped section has a support portion 20a and a flange portion 20b. The edge of the shadow mask 12 is supported at the inner side of the support portion 20a of the frame 20. The inner shield 22 has a peripheral portion 22a which can be fixed to the flange portion 20b of the frame 20.
In the CRT having the above structure, an electron beam emitted from the electron gun 33 is deflected by the deflection yoke 34 and passes through electron beam passing holes formed in the shadow mask 11 to be scanned onto the fluorescent film 11. The electron beam landed on the fluorescent film 11 excites fluorescent material to display a predetermined image.
The frame 20 usually has a thickness of 0.8-1.6 mm to maintain its strength and a material of pure steel is used therefor. Thus, magnetic attraction through the frame 20 increases due to the earth's magnetic field, which has a negative effect on the trajectory of the electron beam at the stage of driving the CRT. Previously, in order to prevent the above problem, a high voltage had to be applied to a degaussing coil (not shown) for removing the effect by the earth's magnetic field. That is, stronger magnetic field is generated by increasing the voltage applied to the degaussing coil and is used to compensate for the earth's magnetic field for the frame 20. However, since this method cannot completely remove the effect by the earth's magnetic field, the trajectory of the electron beam is affected by the earth's magnetic field and thus the quality of image becomes inferior.